1. Field of the Invention;
The invention relates to a method for the production of expandable plastics granulate and equipment and plant for carrying out or using the method.
2. Description of the Prior Art;
An often used foamed plastics is made of polystyrene. In this process granulate of xe2x80x9cexpandable polystyrenexe2x80x9d, EPS, is processed into blocks or molded parts, while in an intermediate step the granulate is prefoamed. EPS may be manufactured by-suspension polymerization. In that process styrene is polymerized in an aqueous phase with the addition of a blowing agent. In this way a beadshaped granulate is produced within a wide range of bead sizes. A disadvantage of this method is that large quantities of water, which must be cleaned, accumulate and that the granulate is usable for the production of the foamed material only within a limited range of sizes so that a considerable part of the produced polymer must be discarded (or recycled).
In another method, which is little suitable for the production of large quantities of EPS, the polystyrene is, after polymerization, impregnated by a blowing agent in pressure vessels or in extruders. The product is cylindrical granulate.
Further information on foamed materials can be found in Ullmanns Encyklopxc3xa4die der technischen Chemie (4th edition, 1981), volume 20, pages 415 to 432 and volume 19, pages 268 and 131.
The aim of the invention is to provide a method of economical production of expandable plastics granulate, for instance of EPS, by which may be produced large quantities without the disadvantages of the known methods. According to this method a plastics melt is impregnated with a fluid blowing agent which is, at elevated pressure within a given pressure region, only partially soluble in the melt.
Large quantities of Eps or another comparable granulate cannot be economically produced by extruders, because a plurality of extruders used in parallel would have to be used. The use of the equipment according to the invention, in which the impregnation of the plastics melt may be carried out in a single apparatus, represents an economical advantage. The teaching of the invention is based substantially on the discovery that large quantities of expendable plastics granulate may be produced in an apparatus only if provisions against segregation of the melt and blowing agents are possible and are made. According to the invention static mixing elements act during the whole course of the process continuously onto the mixture in such a way that segregation is avoided.
Compared with the known methods using extruders, the method according to the invention has the further advantage that much less energyxe2x80x94about one order lessxe2x80x94is needed for the production of expandable plastics granulate. With this advantage is connected a second one, namely that there is a smaller temperature rise during the impregnation and consequently less heat need be dissipated.
By the addition not only of a blowing agent but also several additives to the melt, the quality of the product can be advantageously influenced. As a blowing agent preferably a chlorofluorocarbon or a low-boiling hydrocarbon, particularly pentane, or a mixture of such hydrocarbon, is used. As additives flameproofing agents (compounds of bromine), lubricants (oil, derivatives of stearic acid), dyes, antioxidants softeners or nucleators (for the formation of cells) may be used.